From: Suraj Kurapati <sunaku@...>
Date: 2011-11-16T04:46:55+09:00
Subject: [ruby-core:41065] [ruby-trunk - Feature #5588] add negation flag (v) to Regexp


Issue #5588 has been updated by Suraj Kurapati.


I don't have a use case for the current behavior; it was just the
simplest way to remove non-matching input characters that obstructed
the matching engine.  And I agree with your examples; people would
naturally think of /(?v:...)/ as a glorified form of /[^...]/.

The solution is to change the parse-tree expansion into this:

  /(?v:r)/ => /(?:(?:rN)?.)/

In this manner, the /(?:rN)/ acts as a barrier that only allows
input characters that *do not* match `r` to be matched by the /./.

However, this seems very similar to Tanaka's 2007 solution[1]:

  /(?v:r)/ => /(?:(?!r).)/

I will play with Oniguruma in GDB some more to learn how (?!) works.
Perhaps my OP_NEGATE modification is actually unnecessary.  Cheers.

[1]: http://www.ruby-forum.com/topic/133413#595368
----------------------------------------
Feature #5588: add negation flag (v) to Regexp
http://redmine.ruby-lang.org/issues/5588

Author: Suraj Kurapati
Status: Assigned
Priority: Normal
Assignee: Yui NARUSE
Category: core
Target version: 2.0.0


Please add a negation flag (v) to regexps which inverts them:

  "ruby" =~ /perl/v       #=> true   (turn on negation)
  "ruby" !~ /perl/v       #=> false  (turn on negation)
  "ruby" =~ /(?v:perl)/   #=> true   (turn on negation)
  "ruby" !~ /(?v:perl)/   #=> false  (turn on negation)
  "ruby" =~ /(?-v:perl)/  #=> false  (turn off negation)
  "ruby" !~ /(?-v:perl)/  #=> true   (turn off negation)

The flag name "v" comes from the ex(1) command of the same name.

This has beneficial applications where it is sometimes difficult
to construct what you want to match but much easier to construct
what you *do not* want to match.  Having this negation built in
the regexp itself would remove the need for us to change our 
Ruby code to process a regexp in a different way.  

For example, suppose that you are passing a regexp to the `--name`
command-line option of MiniTest.  This regexp tells MiniTest to
only run those tests whose names match.  If Ruby had a negation
flag on its regexps, then it would be suddenly trivial to make
MiniTest skip certain tests by negating the regexp we pass in.

In this manner, we get a beneficial feature without ever changing
our Ruby code (the codebase of MiniTest in this example).  :-)

Thanks for your consideration.


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